Field
This invention relates generally to a system and method for recovering a symbol clock in a receiver that receives an M-ary pulse position modulation (PPM) signal and, more particularly, to a system and method for recovering a symbol clock in a receiver that receives an M-ary PPM signal, where the method includes comparing a largest magnitude pulse value of groups of pulse values in the PPM signal at consecutive time slots to determine whether the symbol clock identifies the beginning of a symbol group, and if not, changing the symbol clock.
Discussion
Data and communications messages sent between various devices and systems, such as cellular signals, satellite uplink and downlink signals, satellite cross-link signals, etc., are modulated onto a carrier wave at various modulation frequencies. Sometimes these signals are optical signals where light beams are modulated to signify data. These modulated signals often employ a slot clock to regulate the location of individual time slots where changes in the signal can be transmitted. Various techniques are known in the art for effectively recovering the slot clock in the receiver.
M-ary pulse position modulation (PPM) is a form of signal modulation where N message bits (N is equal to the base 2 logarithm of M) are encoded on the signal by transmitting a single signal pulse in one of M possible time-slots. Each signal pulse in the modulated signal is located in any one time slot of a group of M time slots, where the group of M time slots is referred to as a symbol. As the signal is constructed and transmitted, consecutive symbol groups in the signal typically follow each other without gaps therebetween. Therefore, when the signal is received in the receiver, the demodulation technique also needs to produce a symbol clock that identifies the first time slot in each symbol group. However, because the last time slot in one symbol group is adjacent to the first time slot in the next symbol group it is generally difficult to identify where one symbol group ends and the next symbol group begins. More specifically, identifying the boundary between symbol groups from the pulses that are in the symbol groups is difficult because the pulses may be very close to each other or may be far apart, such as being located in the last time slot in one symbol group and being located in the first time slot in the next symbol group or being located in the first time slot in one symbol group and being located in the last time slot in the next symbol group. For example, if the modulation scheme employs 4-ary PPM (M is equal to 4), the signal pulse for any one symbol group can be placed in any one of four time slots in that group, where any of the next four time slots can include the pulse for the next symbol group.
Identification of the true location of the boundary between symbols is a necessary first step to correctly deduce the data bits being indicated by the signal. Confident and rapid identification of this boundary provides benefits to the user in the form of reduced data latency and increased channel efficiency. Although trial and error strategies can be devised that employ examination of recovered data bits for the presence of known patterns (such as frame sync patterns) to indicate successful symbol boundary alignment, it would be clear to those skilled in the art that these strategies increase the time to determine the proper alignment, which increases data latency, and usually discard otherwise useful signal information associated with unsuccessful trials, and thus decrease channel efficiency, and also depend on fixed patterns that carry no data and therefore decrease channel efficiency.